Pressure drop is a phenomenon experienced when fluids are passed through particulate beds of catalyst or sorbent. In extreme cases pressure drop can lead to unacceptable increases in compression and pumping costs. Thus there is a desire to reduce pressure drop when process fluids are passed through reaction vessels containing fixed beds of catalyst or sorbent.
Reaction vessel configurations in which the pressure drop is reduced compared to simple axial flow reactors are known. Radial-flow reactors, for example as described in U.S. Pat. No. 4,033,727, typically comprise a cylindrical central conduit surrounded by an annular cylindrical catalyst bed bounded by perforate catalyst restraining means defining an annular cylindrical void between the catalyst bed and the interior of the vessel wall. The complexity of such radial flow reactor designs can make them expensive and difficult to install.
Opposed-axial flow reaction vessels offer an operator of a process wherein a process fluid is passed axially through a bed of catalyst or sorbent disposed within a vessel, the ability to reduce the pressure drop through the catalyst to approximately one-eighth of that obtained where the process fluid is conducted through the whole length of the catalyst bed.
EP 075056 describes a split axial flow converter for the low-pressure synthesis of ammonia. The converter comprises a reaction vessel containing an ammonia synthesis catalyst with opposed first and second inlet ports and gas collection means disposed approximately centrally within the catalyst bed. The gas collection means comprise a number of perforate concentric rings bisected by an outlet pipe that exits the reactor radially at the level of the concentric rings. Such a design, while effective in reducing pressure drop uses gas collection means that are unsuitable for fitting to an existing reactor and are of limited versatility. In particular the outlet pipe extending radially through the reactor wall reduces the pressure-bearing ability of the reaction vessel and its installation into an existing vessel presents considerable difficulties in stress-relieving the vessel prior to use.
GB 1307845 describes an ammonia or methanol synthesis reactor comprising a catalyst bed space defined by two coaxial hollow cylinders, the inner cylinder comprising two portions one of which is greater in diameter than the other and a heat exchanger occupying at least part of the portion of greater diameter. In a preferred form of the reactor the outlet of the cold side of the heat exchanger is in flow communication with separated inlets at the top and bottom of the catalyst bed so that the stream of incoming gas leaving the heat exchanger is divided, one part being led to the top of the catalyst bed, the other part being led to the bottom of the catalyst bed, whereafter the two flows meet and leave by a bed outlet disposed in the catalyst bed at an intermediate position which may be near to the inlet to the ‘hot’ side of the heat exchanger. Such a design is complex, difficult to fabricate and is unsuitable for fitting to an existing reactor.
Thus there is a need for a method to adapt an axial flow reaction vessel to an opposed flow reaction vessel that comprises providing process fluid collection means that are simple to fabricate and which utilizes existing inlet and outlet ports.